Carburetors which feed fuel to general-purpose engines feed very small fuel flow quantities, compared with carburetors which feed fuel to four-cycle engines, such as automobile engines, and there are great change ratios in the fuel flow quantity, which depend on deviations in the positions or dimensions of various parts or differences in the quality of the parts. In addition, there are differences in the performances of the engines to which the fuel is fed. Therefore, the fuel flow quantity must be adjusted individually.
Therefore, manual adjustment valves, which are made so that the fuel flow quantities can be adjusted individually, are placed in the main fuel path and the low-speed fuel path. These adjustment valves consist of a needle-shaped valve body which is inserted in the fuel path and continuously changes its effective area, a screw part which is joined with the main body of the carburetor by screwing and can be moved forward and backward while this valve body is rotated, and a head part, which is exposed outside of the main carburetor body and has the purpose of rotating the screw part.
These adjustment valves are supplied to general users in a state in which the adjustment operations are fundamentally performed by the manufacturers of the carburetors and the engines or the machinery in which they are assembled, and they are adjusted to the optimum fuel flow quantities. However, the valves may be operated in cases in which the users think that the performances of the engines need to be maintained in response to changes in the places or conditions in which the users use them, for example, changes in air pressure, or they think that temporary malfunctions in the engines need to be corrected or their performances improved. As a result of the carburetor being adjusted outside its range of adjustment and the air-fuel mixture being made too lean or too rich, troubles such as a reduction of output, a worsening of the state of the exhaust, and engine stoppage can be produced.
On the other hand, exhaust regulations are now being imposed even on general-purpose engines; therefore, the adjustment valves which manufacturers have adjusted within ranges which conform to these exhaust regulations are required to have adjustment devices which impose means of limitation in such a way that users cannot adjust them outside these conformance ranges.
As an adjustment device which has a means of limitation such that the adjustment valves in such carburetors cannot be operated above a certain point, Japanese Patent No. Showa 47-42424 (Patent Reference 1), for example, discloses a limit cap which has an arm which projects in a radial direction and is installed on the head of the adjustment valve; the valve can only be operated within one rotation due to its touching the main carburetor body, which is a stopper.
Moreover, in Japanese Unexamined Patent Application No. Showa 61-134555 (Patent Reference 2), for example, an adjustment device is disclosed in which the adjustment valves in the main fuel path and the low-speed fuel path are placed close to and parallel with each other, and they can be only operated within one rotation due to an arm on each one touching the other adjustment valve, which is a stopper, or a limit cap.
In these conventional adjustment devices, the limit caps are temporarily fitted to such a degree that they do not drop off the head before the manufacturer adjusts them, and the main fitting is done in a position such that the arm touches the stopper when the adjustment is finished, or they are fitted in a position in which the arm touches the stopper after the manufacturer's adjustment, without a temporary fitting.
However, the limit caps which are a means of limiting these rotations are exposed to the outside of the main carburetor body. Therefore, they have the problems that they are easily removed by intentional vandalism of the users, or the adjustments may slip due to the user unintentionally touching the device during the adjustment.
Therefore, for example, in the pilot screw (adjustment valve) adjustment device for carburetors which is shown in Japanese Utility Model No. Heisei 6-40339 (Patent Reference 3), as shown in FIGS. 14A and 14B, the screw part of the adjustment valve 1a which adjusts the effective opening area of the fuel flow path is attached by screwing it into the adjustment hole 3a of the main carburetor body 2, and an adjustment tool is made so that it can be inserted into the tool insertion groove 11a of the adjustment valve 1a through the regulating hole 41a of the regulating member 4a. 
By rotating the adjustment tool inserted into the tool insertion groove 11a, the adjustment valve 1a is rotated, and the effective opening area of the fuel flow path is variably adjusted by rotating the adjustment valve 1a. Thus, the rotation of the adjustment valve 1a is limited to a specific adjustment range by the fact that the side surface of a semicircular flange part touches the locking part 42a of the regulating member 4a. 
In the adjustment device of this adjustment valve, the adjustment valve 1a and the regulating member 4a are disposed in a sleeve 5a. In a carburetor which has a regulating valve of the fuel flow quantity and a control means for this regulating valve, it can be made difficult to remove the device by the user's intentional vandalism performed on the aforementioned regulating member which is disposed in this sleeve.
However, the adjustment device of the adjustment valve in this carburetor uses a flat-plate-shaped regulating member as the means of control, and the regulating member limits removal by being inserted into and fixed in a storage hole. However, since it is formed as a flat plate, it must be pressed in or fastened, or, for example, fixed by using a separate means of fixing, such as a screw, when it is pressed into and fixed in the storage hole.
Therefore, it is difficult to make the limit cap small and the parts are costly. Furthermore, it has problems in its operation, namely, that it is hard to make it difficult for users to remove it, and it is easily removed during adjustment, etc.
Documents of the Related Art    1. Japanese Patent No. 47-42424    2. Japanese Unexamined Patent Application No. 61-134555    3. Japanese Unexamined Patent Application No. 6-40339